1. Technical Field
The present invention relates to a digital controlled oscillator and a frequency variable oscillator which generate a signal having a desired frequency by using a ring delay circuit in which a plurality of delay element are connected in a ring shape.
2. Related Art
Conventionally, a digital controlled oscillator is known which generates an output pulse signal having a desired frequency by using a ring delay circuit (ring oscillator) in which a plurality of delay elements are connected in a ring shape (e.g. see JP-A-7-183800).
Specifically, the digital controlled oscillator uses a counter which counts the number of circulations of a pulse signal in the ring delay circuit and a selection circuit which selects any one of outputs of the delay elements (specifies a circulation position of a pulse in the ring delay circuit) to change a signal level of an output pulse signal by using a signal selected by the selection circuit every time when the pulse signal passes through multiple stages of delay elements, the number of the stages corresponding to the desired frequency (period).
However, according to the above conventional oscillator, the inverse of delay time in the delay elements configuring the ring delay circuit is frequency resolution of the output pulse signal. Hence, frequency resolution higher than the inverse of the delay time cannot be realized.